The Flesch-Kincaid Readability Test

One of the most important things that writing teachers emphasize to their students is for the students to “know their audience.” True enough. It wouldn’t make sense to use college-level writing to write a high school textbook, and it would be unthinkable to write a doctorate-level dissertation using third-grade language. In order to effectively get the message across, it is important for the writer to learn how to adjust the readability level of what they write. Unknown to many, there is a formula to determine a text’s readability: the Flesch–Kincaid Readability Test.

Exploring the Roots of the Flesch-Kincaid Readability Test

Before the Flesch-Kincaid Readability Test was developed, there was the Flesch Reading Ease formula.  Now considered the grandfather of all readability formulas, it was developed in 1948 by Rudolph Flesch, an Austrian author and writing consultant. His body of work reflected his belief that phonics should be the underlying basis of readability. The idea took off when his now-landmark article, “A New Readability Yardstick,” was published in the 1948 edition of the Journal of Applied Psychology.

The article put forward a mathematical system that determined the readability of a document based on the average sentence length and the average number of syllables per word. It then assigned a score ranging from zero, meaning that the text is very confusing, to 100, which means that the text very easy to understand. If a text scores around 80, it means that the text can be read at the 6th or 7th-grade levels.

However, the Flesch Reading Ease Formula was not without limitations. Some of the word counting rules were very ambiguous (for example, vowels automatically count as one syllable, and endings such as –ed are ignored).  Not to mention, as well, that at the time it was developed, words and syllables all had to be counted by hand. Lastly, the score produced had to then be filtered through a chart in order to find the text’s assigned reading level.

Building off a Predecessor: How Kincaid Improved on Flesch’s Earlier Work

In the mid-70s, J. Peter Kincaid, an American scientist and educator, was contracted by the US Navy to revisit the Flesch formula and devise a brand-new measure of readability. Utilizing his background in psychology, Kincaid made several changes to the Flesch Reading Ease Formula, eventually coming up with this:

0.39 (total words / total sentences) + 11.8 (total syllables / total words) – 15.59

Instead of a numerical score that had to be interpreted, the test resulted in a figure that corresponded to a U.S. grade level.  If the text, for example, was suitable for a 6th grader, then the final numerical score at the end of the formula would be around 6. Thus, the Flesch-Kincaid Readability Test was born.

The test was immediately utilized by the US Navy to evaluate the reading difficulty of their technical manuals. After its successful initial implementation, it became so ubiquitous in the armed forces that it eventually became “United States Military Standard.”

It has spread through other industries since and percolated through every imaginable activity that requires evaluating text for readability. Most notably, the Flesch-Kincaid Readability Test was adopted by the education sector and is now used by teachers, librarians, and even parents in choosing reading material for their children. The test was also adopted by the state of Pennsylvania where automobile insurance policies were required to be written at a ninth-grade readability level.

The Practical Uses of a Readability Score In the Age of the Internet

In the age of the internet, what role does the Flesch-Kincaid Readability Test play when creating content?

Short answer: everything. For content creators, they should be aware that the average American adult is able to comprehend text at a 7th to 9th-grade level. This means that creators need to tailor their verbiage and sentence lengths to cater to that of the average reader, leaving out unnecessarily complex words and cutting long sentences into more concise bits. Not doing so may result in the alienation of readers.

Online content creators can take advantage of the Flesch-Kincaid Readability Test in a variety of activities, such as stringing website copies, writing product advertisements, or boosting SEO performance. This is because as research has shown that a piece of online content’s readability has a positive (or negative) correlation with the engagement of its target audience.

For those who are managing content-driven websites and profiting off audience engagement, using the Flesch-Kincaid Readability Test in curating content may result in a number of measurable results, including but not limited to:

  • lengthening the duration a reader spends time on the site
  • reducing the bounce rate
  • increasing the number of shares for a particular piece
  • increasing the instances of clicks to call-to-action blurbs
  • increasing the rate of adding products to cart
  • increasing the number of returning readers

To the convenience of content managers, a number of online websites have developed their own online calculators that make use of the Flesch-Kincaid Readability Test formula to come up with a readability score without having to crunch the numbers manually. All that needs to be done is copy the block of text and paste it on their website and in a few seconds, a score is generated. Some SEO tools, such as Yoast or our headline analyzer, even provide the calculated Flesch-Kincaid so that you know how readable your titles and text are.


Most people have not heard of this test, but all of us use it to some degree. Many of the websites we visit use it, the books we select at the library for our children use it, and even the magazine and newspaper articles we read use it. It has a large impact on our lives, and helps create content that can reach broad audiences, as well as narrow, focused groups, making it one of the most versatile and useful tools available to us today.

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